Here's some info which came with my boat: it appears to be an extract from a UK yacht guide.
(Note that the first boat, the L32, is not an LM; and not included are LM’s Mermaid models, though they are mentioned on this page.)
I was unaware of the LM81, which appears to be a non-wheelhouse version of the LM26, with a more conventional yacht layout below. It sports a fixed windscreen (as per Hallberg Rassy and other Scandinavian yachts).
I was also interested to read that the LM27 was a development of the LM24, rather than the other way around as I’d assumed.
I'm trying, for the first time, to show scanned images of the LM brochures that were sent to me about 20 years ago by the US Importer, Anchor Marine. He gave me permission to copy the originals then return them to him, which I did. The copies aren't the greatest quality from back then. I just scanned and uploaded them to Google Photos. Click on the link, then each pic to enlarge. If you want to enlarge the pic more, select the magnifying glass icon in the upper right of the photo. There's also a slider icon in the upper right, after selecting the magnifying glass, if you'd like to enlarge it more to read the text. Hope this works.
I would be pleased, but surprised, if anyone had the original circuit diagrams for the whole boat.
The wiring of the engine and its instruments and controls is probably the most complex wiring on the boat, and details of this will be found in the workshop manual for the particular engine (which can probably be found online). My boat has a Bukh DV36. There are several different wiring diagrams for that in the DV36 workshop manual, according to which control/instrument l panel etc. is fitted. I find my wiring is not exactly the same as any single one of them, but a hybrid between 2 or 3 of those depicted in the manual.
Other than that, the original wiring was, I believe, fairly basic and simple, and should be fairly easy to deduce.
The complications are likely to come in the form of adaptions and additions (and bodges!) that have been made by subsequent owners.
If you have any particular questions about the wiring, people on this forum may well be able to help. I suggest you post such questions under the 'Problems and Solutions > Advice Sought' section of the forum, rather than in this thread.